The present invention relates to a semiconductor device having an insulating, gate using a nitride semiconductor for an active layer thereof and to a method for fabricating the same.
FIG. 19 shows a cross-sectional structure of a conventional Schottky-gate field effect transistor (FET) which is composed of Group III-V nitride semiconductors.
As shown in FIG. 19, a channel layer 102 composed of gallium nitride (GaN) and a carrier supply layer 103 composed of n-type aluminum gallium nitride (AlGaN) are formed successively on a substrate 101 composed of sapphire. A two-dimensional electron gas layer composed of a potential well and having an extremely high electron mobility is formed adjacent the heterojunction between the upper portion of the channel layer 102 and the carrier supply layer 103. For this reason, the FET is also termed a high electron mobility transistor (HEMT).
In the foregoing conventional Schottky-gate FET, however, the breakdown voltage of the gate electrode is determined by the Schottky characteristic thereof so that the reverse breakdown voltage of the gate electrode is limited. In addition, a forward voltage applied to the gate electrode is also limited to about only 2 V so that a high-output semiconductor device (power device) having a high current driving ability is not obtainable.